Environmentally isolated waveguide display

ABSTRACT

A waveguide display having an input image generator providing image light projected over a field of view; a waveguide having first and second external surfaces; and at least one grating optically coupled to the waveguide for extracting light towards a viewer. The waveguide has a lateral refractive index variation between said external surfaces that prevents any ray propagated within the waveguide from optically interacting with at least one of the external surfaces.

PRIORITY CLAIMS

This application claims priority from U.S. Provisional Application Ser.No. 62/125,066 entitled OPTICAL WAVEGUIDE DISPLAYS FOR INTEGRATION INWINDOWS filed on 12 Jan. 2015, which is hereby incorporated by referencein its entirety.

CROSS REFERENCE TO RELATED APPLICATIONS

The following patent applications are incorporated by reference hereinin their entireties: U.S. Pat. No. 6,115,152 entitled HOLOGRAPHICILLUMINATION SYSTEM, PCT Application No. US2006/043938, entitled METHODAND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY, PCT Application No.PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMERDISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, PCT Application No.GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S. Pat. No. 8,233,204entitled OPTICAL DISPLAYS, U.S. patent application Ser. No. 13/317,468entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, U.S. patentapplication Ser. No. 13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY,and U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENTWAVEGUIDE DISPLAY, PCT Application No. PCT/GB2013/000005 entitledCONTACT IMAGE SENSOR USING SWITCHABLE BRAGG GRATINGS, PCT ApplicationNo. PCT/GB2013/000500 entitled WAVEGUIDE FOR HOMOGENIZING ILLUMINATION,and U.S. Pat. No. 8,224,133 entitled LASER ILLUMINATION DEVICE.

BACKGROUND OF THE INVENTION

This invention relates to a display device, and more particularly to aholographic optical waveguide display.

Optical waveguide devices are being developed for a range of displayapplications such as Head Mounted Displays (HMDs) and Heads Up Displays(HUDs). Another field of application of waveguides is in sensors such aseye trackers such as the ones disclosed in PCT Application No.PCT/GB2014/000197 entitled HOLOGRAPHIC WAVEGUIDE EYE TRACKER by Popovichet al and finger print sensors such as the ones disclosed inPCT/GB2013/000005 entitled CONTACT IMAGE SENSOR USING SWITCHABLE BRAGGGRATINGS by Popovich et al. However, waveguide devices that use totalinternal reflection (TIR) to transmit image information suffer from theproblem the beam propagation may be disturbed by damage to orcontamination of the external waveguide surfaces by foreign materials.There is a requirement for a waveguide display in which image lightpropagated within the waveguide is isolated from the externalenvironment.

SUMMARY OF THE INVENTION

It is a first object of the invention to provide a waveguide display inwhich image light propagated within the waveguide is isolated from theexternal environment.

The objects of the invention are achieved in one embodiment of theinvention in which there is provided a waveguide display comprising: aninput image generator providing image light projected over a field ofview; a waveguide having first and second external surfaces; and atleast one grating optically coupled to the waveguide for extractinglight from the waveguide towards a viewer of the display.

The waveguide has a lateral refractive index variation between theexternal surfaces that prevents rays propagating within the waveguidefrom optically interacting with at least one of the external surfaces.

In one embodiment the waveguide contains a GRIN medium and the gratingis disposed in proximity to one of the external surfaces.

In one embodiment the waveguide contains a GRIN medium, and the gratingis disposed within the GRIN medium.

In one embodiment the waveguide contains a GRIN medium, and the gratingis a surface relief structure formed on one of the external surfaces.

In one embodiment the waveguide comprises a first waveguide portioncontaining a GRIN medium abutting a second waveguide portion operatingin TIR and containing at least one grating for extracting light from thesecond waveguide portion towards a viewer of the display.

In one embodiment the waveguide further comprises an input grating.

In one embodiment the waveguide is immersed in air.

In some embodiments the display provides a HUD, HMD or a light fielddisplay.

In some embodiments the waveguide is curved.

In some embodiments the waveguide comprises at least one GRIN waveguideportion optically coupled to at least one TIR waveguide portion, eachthe TIR waveguide portion containing at least one grating.

In one embodiment the waveguide comprises a stack of GRIN waveguidesoptically coupled to a stack of TIR waveguides, each the TIR waveguidecontaining at least one grating.

In one embodiment the waveguide is immersed in a low refractive indexexternal medium and comprises a high refractive index core sandwiched bya low refractive index clad layer and at least one grating layer. TIRtakes place between the interface of the core layer and the gratinglayer and the interface of the grating layer and the external medium.

In one embodiment the core and the grating layer have substantially sameaverage refractive index.

In one embodiment the core has a refractive index greater than thegrating layer average index.

In one embodiment the apparatus further comprises low refractive indexlayers overlaying at least one of the grating layer and the lowrefractive index clad layer.

In one embodiment the grating layer comprises a grating sandwiched bytransparent substrates, the grating layer and the substrates havingsimilar refractive indices.

In one embodiment the grating layer comprises an input grating and anextraction grating.

In some embodiments the display further comprises a beamsplitter layer.

In some embodiments the image light is collimated.

In some embodiments the grating is a Bragg Grating, a surface reliefgrating or a switchable Bragg grating recorded in a HPDLC material, auniform modulation HPDLC material or a reverse mode HPDLC material.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart illustrates the spatial variation of refractive indexin a GRIN device.

FIG. 2 is a schematic cross section view of a GRIN light guide operatingat infinite conjugates and providing an intermediate focal plane in oneembodiment.

FIG. 3 is a schematic cross section view of a GRIN light guide operatingat infinite conjugates and providing two conjugate intermediate focalplanes in one embodiment.

FIG. 4A is a cross section view illustrating beam paths in a portion ofa planar waveguide.

FIG. 4B is a cross section view illustrating beam paths in a portion ofa curved waveguide.

FIG. 5 is a schematic plan view of a curved GRIN light guide comprisingabutting GRIN elements in one embodiment.

FIG. 6 is a schematic plan view of a near eye displays comprising curvedGRIN light guide and a planar holographic waveguide in one embodiment.

FIG. 7 is a schematic view of a further embodiment of the invention.

FIG. 8 is a schematic view of a further embodiment of the invention.

FIG. 9 is a schematic view of a further embodiment of the invention.

FIG. 10 is a schematic view of a further embodiment of the invention.

FIG. 11 is a schematic view of a further embodiment of the invention.

FIG. 12 is a schematic view of a further embodiment of the invention.

FIG. 13 is a schematic view of a further embodiment of the invention.

FIG. 14A is a schematic view of a further embodiment of the invention.

FIG. 14B is a schematic view of a further embodiment of the invention.

FIG. 15 is a schematic side view of a GRIN waveguide in embodiment ofthe invention.

FIG. 16 is a schematic side view of a GRIN waveguide in embodiment ofthe invention.

FIG. 17 is a schematic side view of a GRIN waveguide in embodiment ofthe invention.

FIG. 18 is a schematic side view of a GRIN waveguide in embodiment ofthe invention.

FIG. 19 is a schematic side view of a GRIN waveguide in embodiment ofthe invention.

FIG. 20 is a schematic side view of a GRIN waveguide in embodiment ofthe invention.

FIG. 21 is a schematic side view of an aberration-correcting GRINwaveguide in embodiment of the invention.

FIG. 22 is a block diagram of an architecture for coupling GRINwaveguides to an image extraction grating waveguide in one embodiment.

FIG. 23 is a block diagram of an architecture for coupling GRINwaveguides to an image extraction grating waveguide in one embodiment.

FIG. 24 is a waveguide comprising a grating layer sandwiched by GRINlayers in one embodiment.

FIG. 25 is a waveguide architecture using GRINs recorded in HPDLC in oneembodiment.

FIG. 26 is a GRIN device comprising concentric annular GRIN layers inone embodiment.

FIG. 27 is a GRIN device comprising concentric annular GRIN layersrecorded in HPDLC in one embodiment.

FIG. 28 is a section of a TIR waveguide device with an externalprotective layer in one embodiment.

FIG. 29 is a section of a TIR waveguide device with an externalprotective layer in one embodiment.

FIG. 30 is a section of a TIR waveguide device with an externalprotective layer in one embodiment.

FIG. 31 is a section of a TIR waveguide device with an externalprotective layer in one embodiment.

FIG. 32 is a waveguide display using an external protective layeraccording to the principles of the invention in one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described by way of example only withreference to the accompanying drawings. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription. For the purposes of explaining the invention well-knownfeatures of optical technology known to those skilled in the art ofoptical design and visual displays have been omitted or simplified inorder not to obscure the basic principles of the invention. Unlessotherwise stated the term “on-axis” in relation to a ray or a beamdirection refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories. Parts of the following description will bepresented using terminology commonly employed by those skilled in theart of optical design. It should also be noted that in the followingdescription of the invention repeated usage of the phrase “in oneembodiment” does not necessarily refer to the same embodiment.

One known solution for conveying images down a waveguide withoutinterference from surface contamination relies on Gradient Index (GRIN)optics. GRIN optics reproduces the optical properties of sphericallenses by gradual varying the refractive index of a material. In themost common application, GRIN fibers, the lens properties result from aradially varying index. Since the optical properties rely on the indexdistraction the input and output faces of a GRIN lens may be planar.GRIN lenses are manufactured by using various methods including neutronirradiation, chemical vapor deposition, ion exchange and partialpolymerization, in which an organic monomer is partially polymerizedusing UV light at varying intensities. By precisely varying theirrefractive index, gradient index lenses are able to continuously bendlight within the lens. This contrasts with conventional sphericallenses, which bend light only twice: when light meets the front surfaceof the lens and when it exits the back of the lens. Gradient indexlenses can be positive (converging) or negative (diverging). GRIN lensesare capable of high quality imaging as demonstrated by their successfulapplication in endoscopes.

FIG. 1 shows typical refractive index profile across a GRIN lens. Theelement has a thickness of 2 mm along the y-direction of the insetCartesian coordinate frame (which will also apply to all of embodimentsto be discussed below). The direction of beam propagation is in thez-direction. The refractive index of the GRIN varies from 1.46 to 1.5.

FIG. 2 shows a GRIN lightguide 10 divided into two regions 11 and 12.The two regions may correspond to separate GRIN elements. The lightguidehas an input surface 13 and intermediate focal plane 14 (which maycorrespond to the interface of two separate GRIN elements) and an outputsurface 15. Input collimated light 1010 having a field of view (FOV) isincident on the input surface. The GRIN focuses the light onto theintermediate focal plane such that the three beams illustrated,1011-1013, form focal spots 1017-1019. The beams are then re-expanded inthe second portion of the light guide and subsequently exit the lightguide as the near-collimated beams 1014-1016 through the exit pupil1020. From consideration of FIG. 2 it should be apparent that byapplying the same optical principle it should be possible to engineerlonger light guides containing more than one intermediate focal plane.FIG. 3, for example, shows how the embodiment of FIG. 2 may be extendedto a light guide containing two intermediate focal planes. A beamcollimator 20 provides collimated light 1024 over a FOV. The four GRINregions 21-24 provide focal planes indicated by 26, 28 with maximum beamexpansion occurring at the planes 25, 27, 29. Three separate beam pathsthrough the light guide are indicated by 1021-1023 with the entrance andexit pupils being indicated by 1024, 1025.

To be of practical use in eye wear a GRIN lightguide should be curved inat least one plane of projection. In one embodiment the curvaturesshould match the profile of a spectacle lens. The inventors propose thatthe GRIN lightguide can be used to overcome the problem encountered whencurved lightguide designs are attempted using total internal reflection(TIR) waveguides. The nature of the problem is illustrated in FIG. 4which compares beam propagation in small sections of a planar waveguide(FIG. 4A) and a curved waveguide (FIG. 4B). The beams in the planarwaveguide are well separated as shown by the illustrated beam paths1030, 1031. In the case of the curved waveguide element the beams pathsare mixed leading to scrambling of the waveguide pupil. Unfortunately,the disorder cannot be corrected by modifying the curvatures of thewaveguide surfaces. There may be some scope for compensation based incurving the input and output surfaces. However the number ofoptimisation degrees of freedom provided by the surface curvatures willbe insufficient for most display applications.

FIG. 5 shows one embodiment of the invention in which a curved waveguide40 comprises abutting or daisy-chaining GRIN elements. To achieve asmooth curve the input and output surfaces of the elements should have asmall wedge angle (or may have parallel input and output surfaces butsandwich a wedge-shaped layer of index-matching material. Multiple focalsurfaces are formed as in the embodiments of FIG. 3. Advantageously, theinput is collimated and should fill the input pupil to fill theintermediate pupils within in and at the output of the waveguide. Insome embodiments it may use non-collimated light.

In one embodiment shown in FIG. 6 the invention provides a near eyedisplay 50 comprising a GRIN lightguide 40, identical to the one in FIG.5, optically coupled to an image extraction waveguide 51 for directingthe image light to the eye 53 of the viewer. In contrast to the GRINlight guide the image extraction waveguide transmits image light bytotal internal reflection (TIR) as indicated by the ray path 1051-1053.The waveguide essentially comprises a holographic grating 52 sandwichedbetween transparent optical substrates. The grating is lossy, that is,it has diffraction efficiency (DE) varying from a low value at the endof the grating nearest the GRIN lightguide to a high value at its otherend. The effect of the varying DE is to provide uniform light extractionalong the length of the grating thereby expanding the exit pupil oreyebox of the display as indicated by the rays 1054-1056. Coupling ofthe GRIN light guide and the waveguide may be accomplished in severaldifferent ways. In one embodiment the two devices are coupled end-to-endas indicated in FIG. 6. In this case it will be necessary to engineer asharp bend to ensure that the entire angular image content 1050 emergingfrom the GRIN lightguide enters a TIR state in the waveguide. Thelagging ray angle would need to be just above the TIR angle (>41.5 degin glass). In another embodiment the required beam steering may beprovide by a coupling prism. The disadvantage of this approach is thatit would entails a step from the lightguide to the holographicwaveguide. In another embodiment the prism may be replaced by a grating.The precise details of the coupling interface will depend on theapplication. For example, ergonomic constraints on the maximum bend,waveguide thickness, field of view and other parameters will have animpact on the relative ease of implements of the various solutions. Inone embodiment the apparatus of FIG. 6 is embedded in one or both of thecurved eyepieces of a pair of prescription glasses. In one embodimentthe apparatus of FIG. 6 is embedded within curved, emmetropic (that is,zero prescription), eyewear such as sunglasses).

The image extraction waveguide in FIG. 6 may be based on a passiveholographic technology. Alternatively, it may use switching gratingtechnology such as Switchable Bragg Gratings (SBGs). The advantage ofswitching gratings is that they allow the field of view to be expandedusing tiling. In the light of current waveguide fabrication limitations,it is likely the waveguide will be need to be a planar element. However,the invention is equally applicable to curved waveguides. The waveguidemay be based on any of the holographic waveguide embodiments disclosedin U.S. Pat. No. 8,233,204 entitled OPTICAL DISPLAYS, U.S. patentapplication Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY,PCT Application No. GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S.patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATEDEYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitledHOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No.13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY all of which areincorporated herein by reference in their entireties.

Holographic waveguides based on Switchable Bragg Gratings (SBGs). SBGsare fabricated by first placing a thin film of a mixture ofphotopolymerizable monomers and liquid crystal material between parallelglass plates. One or both glass plates support electrodes, typicallytransparent indium tin oxide films, for applying an electric fieldacross the film. A volume phase grating is then recorded by illuminatingthe liquid material (often referred to as the syrup) with two mutuallycoherent laser beams, which interfere to form a slanted fringe gratingstructure. During the recording process, the monomers polymerize and themixture undergoes a phase separation, creating regions densely populatedby liquid crystal micro-droplets, interspersed with regions of clearpolymer. The alternating liquid crystal-rich and liquid crystal-depletedregions form the fringe planes of the grating. The resulting volumephase grating can exhibit very high diffraction efficiency, which may becontrolled by the magnitude of the electric field applied across thefilm. When an electric field is applied to the grating via transparentelectrodes, the natural orientation of the LC droplets is changedcausing the refractive index modulation of the fringes to reduce and thehologram diffraction efficiency to drop to very low levels. Note thatthe diffraction efficiency of the device can be adjusted, by means ofthe applied voltage, over a continuous range. The device exhibits near100% efficiency with no voltage applied and essentially zero efficiencywith a sufficiently high voltage applied. In certain types of HPDLCdevices magnetic fields may be used to control the LC orientation. Incertain types of HPDLC phase separation of the LC material from thepolymer may be accomplished to such a degree that no discernible dropletstructure results. SBGs may be used to provide transmission orreflection gratings for free space applications. In waveguideapplications the parallel glass plates used to form the HPDLC cellprovide a total internal reflection (TIR) light guiding structure. Lightis “coupled” out of the SBG when the switchable grating diffracts thelight at an angle beyond the TIR condition. Typically, the HPDLC used inSBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, andcoinitiators. The mixture frequently includes a surfactant. The patentand scientific literature contains many examples of material systems andprocesses that may be used to fabricate SBGs. Two fundamental patentsare: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452by Tanaka et al. Both filings describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices. One of theknown attributes of transmission SBGs is that the LC molecules tend toalign normal to the grating fringe planes. The effect of the LC moleculealignment is that transmission SBGs efficiently diffract P polarizedlight (ie light with the polarization vector in the plane of incidence)but have lower diffraction efficiency for S polarized light (ie lightwith the polarization vector normal to the plane of incidence.

The techniques for designing GRIN light guides should be well known tothose skilled in the art and have been implemented in design softwaresuch as ZEMAX® (ZEMAX Development Corporation, Bellevue, Wash.). GRINssuitable for used with the invention are likely to require a polynomialprofile which may also require additional odd-order polynomial terms tocorrect for the effects of lightguide curvature. Potential issues to beaddressed in reducing the invention to practice include the limitednumber of degrees of freedom available for optimizing the design, theimpact of curvature on aberrations, the tolerances of refractive indexprofile (and impact on relay functionality) and identifying the mostefficient optical means for coupling the GRIN light guide to the imageextraction waveguide

In many practical embodiments the GRIN lightguide will have unitymagnification. However, the invention does not assume any particularmagnification. In one embodiment at least one change in magnification isprovided along the light guide.

In one embodiment illustrated in FIG. 7 there is provided an afocalcylindrical GRIN waveguide element 60 suitable for use in a curvedwaveguide. The element comprises the back to back elements 61 and 62which share a focal surface 63. The input and output surfaces of thewaveguide element are indicated by 63, 64. In practice the input andoutput surfaces will be wedged to give the input and output surfaces 65,67 where 65 is the output surface of the refractive element 66. In theparticular embodiment illustrated the tilt angle will be equivalent to afield angle of ±15 degrees in air or ±9.48 degrees in glass. The tiltangle applied to each end of the waveguide matches the half field anglesof the transmitted image, that is, the maximum design angles that can betransmitted by the waveguide. It should be apparent from considerationof the drawing that the relay therefore has a magnification of exactlyunity. The positive field angles entering the relay exactly match thenegative field angles at the re-imaged pupil. It should also be apparentthat elements of the type illustrated can be daisy-chained to yield aprogressing curve shape with the tilted surfaces 65, 67 providing theinterfaces between adjacent waveguide elements. Typically, a straightGRIN element would use a form such as n(x)=A₀+A₂x²+A₄x⁴+A₆x⁶. Odd termsare added to this expression when tilt is introduced. At the abovestated angles the wavefront errors are found to be negligible.Typically, the GRIN refractive index variation centre to edge is around0.12.

In the embodiment of FIG. 8 there is provided a waveguide based on theelement shown in FIG. 7. Three such elements 70-72 are separated byintermediate, substantially afocal GRIN waveguides 74, 75 of a secondprescription. Collimated input 1070 and output 1071 light is shown. Theapproximate dimensions of the complete waveguide are 68 mm horizontal by36.5 mm. vertical. The waveguide is approximately 3 mm. thick. Thecollimated output light emerging from the end of the waveguide may becoupled in a grating waveguide for image extraction. The waveguide isshown in side projection in FIG. 9.

In the embodiment shown in FIG. 10 the GRIN prescription comprises aneven polynomial up to sixth order coefficients, a radial offset term anda GRIN input tilt term. The latter is effectively an offset to theradial centre of rotation labelled by 1098. It should be noted that thisrelay lens contains two internal focus points. The first of these focuspoints is formed as the light TIRs within the waveguide. The relay issymmetrical about the line bisection the extended input and outputsurfaces 91, 92 in the plane of the drawing. In general, the detaileddesign of a GRIN waveguide must take into account skew rays, dispersion,number of GRIN index steps. Moreover, the GRIN must be manufactured totight tolerances. Errors in the waveguide length would result in theoutput beams being imprecisely collimated. Tolerances of this naturecould be mitigated either by focus adjustment on the input beam or bytrimming the waveguide to the specific length required.

In the embodiment of FIG. 11 a GRIN waveguide 100 provides finiteconjugate 1:1 image imaging between the tilt surfaces 104, 105. Thewaveguide may be divided into the two symmetrical portions 101, 102abutting at the surface 103 where the beam is substantially collimated.The ray paths for one pair of conjugate points are indication by 1100,1101. The waveguide is shown in plan view 110 on FIG. 12. Here a small±2 mm. pupil relay is shown. Note that the relayed surfaces 114, 115 arenon planar. This pupil aberration does not need to be corrected in thecase of a 1:1 relay because it is arranged to be symmetrical about thestop at the surface 103. Three beams 1110-1112 are illustrated.

In one embodiment there is provided a waveguide suitable This isillustrated in FIG. 13 which shows a waveguide 120 for propagating beamsfrom a multiplicity of points labelled by symbols A-C to a multiplicityof points labelled by the symbols A′-C′ Note that the GRIN profile isradial; there is no index variation and hence the image relay process isinvariant along the axis of propagation. The GRIN prescription containsthe series of tilt surfaces labelled by numerals 121-129. In oneembodiment the tilts are symmetrical about the surface 125. Beam paths1120-1122 from the points A-C to A′-C′ are illustrated. Only the upperhalf of the GRIN is used. Hence referring to the reference plane 1123the points A-C could correspond to a first image plane and the pointsA′-C′ are light in proximity to a grating layer for extracting lightfrom the waveguide. An example of grating coupling to an from a GRINwaveguide is provided by the embodiment of FIG. 14 which shows a GRINwaveguide 130 immersed in air 131, comprising a GRIN waveguide 132similar in concept to the one of FIG. 13, an input grating 133 and anoutput grating 134. The input and output gratings are design from 0degrees in air coupling to 75 degrees in glass. Referring to FIG. 14A,the input beam is indicated by 1130, the GRIN guided light by the rays1131 and the output light by 1132. FIG. 14B is a plan view of the raytrace showing the GRIN-guided beam paths 1134, 1135.

We next consider a series of GRIN waveguide embodiments using gratingsto extract collimate image light from the waveguide over a specifiedfield of view. In the embodiment of FIG. 15 a GRIN waveguide 140comprises a planar GRIN substrate 141 having an output surface 142 andan input surface 143 which is also an intermediate focal surface. Firstand second ray paths through the GRIN medium are generally indicated by1140 and 1141. The collimated light beams refract out of the outputsurface as generally indicated by 1142, 1143.

In the embodiment of FIG. 16 a GRIN waveguide 150 comprises a curvedGRIN substrate 151 having a curved output surface 152 and a curved inputsurface 153 which is also an intermediate focal surface. First andsecond ray paths through the GRIN medium are generally indicated by 1150and 1151. The collimated light beams refract out of the output surfaceas generally indicated by 1152, 1153.

In the embodiment of FIG. 17 a GRIN waveguide 160 comprises a curvedGRIN substrate 161 having a curved output surface 162 which has the formof a surface relief grating, and a curved input surface 163 which isalso an intermediate focal surface. First and second ray paths throughthe GRIN medium are generally indicated by 1160 and 1161. The collimatedlight beams refract out of the output surface as generally indicated by1162, 1163.

In the embodiment of FIG. 18 a GRIN waveguide 170 comprises a curvedGRIN substrate 171 having a curved output surface 172 which has the formof a surface relief grating, and a curved input surface 173, which hasthe form of a surface relief grating and which is also an intermediatefocal surface. First and second ray paths through the GRIN medium aregenerally indicated by 1170 and 1171. The collimated light beams refractout of the output surface as generally indicated by 1172, 1173.

In the embodiment of FIG. 19 a GRIN waveguide 180 comprises a curvedGRIN substrate 181 having a curved output surface 182 and a curved inputsurface 183 which has the form of a surface relief grating and which isalso an intermediate focal surface. A curved grating 184 is disposedinside the waveguide in proximity to the output surface. In oneembodiment the grating is a volume or Bragg grating. In one embodimentthe grating is a SBG. First and second ray paths through the GRIN mediumare generally indicated by 1180 and 1181. The collimated light beamsrefract out of the output surface as generally indicated by 1182, 1183.

In the embodiment of FIG. 20 a GRIN waveguide 190 comprises a curvedGRIN substrate 191 having a curved output surface 192 and a curved inputsurface 133 which is also an intermediate focal surface. A curvedgrating 194 is disposed inside the waveguide in proximity to the inputsurface. In one embodiment the grating is a volume or Bragg grating. Inone embodiment the grating is a SBG. First and second ray paths throughthe GRIN medium are generally indicated by 1190 and 1191. The collimatedlight beams refract out of the output surface as generally indicated by1192, 1193.

It should be apparent from consideration of the preceding embodiments ofFIGS. 15-20 that other embodiments may be devised for the purposes ofextracting collimate image light from the waveguide over a specifiedfield of view by using different combinations of curved surfaces andgratings. In some cases multiple layers of gratings may be used for thepurposes of increasing the field of view and propagating red green andblue image content according to the teaching of the above-cited relatedpatent applications.

In one embodiment illustrated in FIG. 21 a waveguide comprises two GRINwaveguide elements arrange in series in which the second elementcorrects the aberrations of the input image light. The input imagelight, which is represented by the curved wavefront 1200, propagatesdown the first waveguide element 241 forming a wavefront 1201 inproximity to the boundary surface 244. After refraction through theboundary surface the refracted wavefront 1202 propagates through thesecond waveguide element and is refracted out of the waveguides as theplanar wave 1203.

In one embodiment there is provided a general waveguide architecturewhich is schematically illustrated by the block diagram of FIG. 22. TheGRIN waveguides 251-253 are coupled into a waveguide 254 containing animage extraction grating by coupling means symbolically indicated by1210-1212. The output from the image extraction waveguide comprisescollimated light in the field of view regions or tiles labelled1213-1215. The coupling means are base on the teachings of the abovedescribed embodiments.

In one embodiment there is provided a general waveguide architecturewhich is schematically illustrated by the block diagram of FIG. 23. TheGRIN waveguides 261-262 are coupled into waveguides 264-266 eachcontaining an image extraction grating by coupling means symbolicallyindicated by 1220-1222. The output from the image extraction waveguidecomprises collimated light in the field of view regions or tileslabelled 1223-1225.

In one embodiment illustrated in FIG. 24 there is provided a waveguide230 comprising at least one grating layer 231 sandwiched by first andsecond GRIN layers 231, 232. In one embodiment the grating is a volumeor Bragg grating. In one embodiment the grating is a SBG.

In one embodiment illustrated in FIG. 25 there is provided a waveguide200 comprising GRIN layers formed in a HPDLC material sandwiched bytransparent electrodes. In the example shown the electrodes layers 201,202 sandwich the stack of GRIN layers 201-203.

In one embodiment illustrated in FIG. 26 a GRIN structure 210 comprisescylindrical concentric GRIN layers 211-214. The structure may be trimmedto form curved substrates or lens elements.

In one embodiment illustrated in FIG. 27 a GRIN structure 220 comprisescylindrical concentric GRIN layers 222-224 sandwiched by transparentelectrodes 221, 225 The structure may be trimmed to form curvedsubstrates or lens elements.

In one embodiment of the invention the refractive index of the GRINvaries radially and along the length of the waveguide.

GRIN waveguides several key advantages over TIR waveguides as used inthe above-cited related patent applications. The first one is the GRINeliminates the problem of banding. A major design challenge in waveguideoptics is coupling the image content into the waveguide efficiently andin such a way the waveguide image is free from chromatic dispersion andbrightness non uniformity. To overcome chromatic dispersion and toachieve the best possible collimation it is desirable to use lasers.However, lasers and other narrow band sources such as LEDs suffer fromthe problem of pupil banding artifacts which manifest themselves asoutput illumination non uniformity. Banding artifacts are formed whenthe collimated pupil is replicated (expanded) in a TIR waveguide. Invery basic terms the light beams diffracted out of the waveguide eachtime the beam interacts with the grating have gaps or overlaps. Thisleads to an illumination ripple. The degree of ripple is a function offield angle, waveguide thickness, and aperture thickness. The effectsare therefore most noticed in narrowband (e.g. laser) illuminationsources. Banding can be smoothed by dispersion with broadband sourcessuch as LEDs. However, current LEDs do not provide enough light outputfor waveguide displays. A second major benefit of GRIN waveguides isthat the guided beams do not interact with the faces of the waveguidemaking the waveguides immune to external contaminants. Additionaloptical layers may be applied without interfering with the waveguiding.The third major benefit of GRIN is that curved waveguides can beengineered much more easily than with TIR waveguides.

Notwithstanding the above advantages of GRINs, in many applications asimilar degree of protection may be provided by using a protectivecladding applied to an exterior surface of the waveguide. Examples ofsuch embodiments of the invention are shown in FIGS. 28-31. In each casethe lower surface of the waveguide is nearest the viewer of the displayand is in contact with air. In each case the uppermost layer isolatesthe TIR from the effects of windscreen damage or contamination andallows additional coatings to be applied to the exterior surface ifrequired. In general, a waveguide must have a core of high indexsandwiched by lower index (clad) layers, one of which may be air.However, if the TIR angles in the core are large enough low indexrefractive materials may be used.

In the embodiment shown in FIG. 28 the display comprises the externalclad layer 230, providing a protective layer, a core layer 231 and agrating layer divided into an input grating 232 and a lossy outputgrating 233 which extracts light uniformly along the waveguide toprovide an expanding exit pupil. The grating layer has an averagerefractive index substantially identical to that of the core. A typicalray path 1240-1242 is shown. An input ray is coupled into a TIR pathinside the waveguide by the input grating and is eventually coupled outof the waveguide by the output grating.

The embodiment of FIG. 29 is identical to the one of FIG. 28 but with anadditional outer clad layer. Note this can be done without penalty asthe inner core satisfies the waveguiding index requirement. The displaycomprises an outer clad 240, providing a protective layer, the cladlayer 241 a core layer 242 and a grating layer divided into an inputgrating 243 and a lossy output grating 244. The grating layer has anaverage refractive index substantially identical to that of the core. Atypical ray path 1250-1252 is shown.

In the embodiment of FIG. 30 the input and output grating layer mediahave a refractive index higher than that of the core. Since the cladlayer has an index lower than that of the core it therefore allows TIR.The display comprises the external clad layer 250, providing aprotective layer, a core layer 251 and a grating layer divided into aninput grating 252 and a lossy output grating 253. A typical ray path1260-1262 is shown.

Note that the grating layers in the above embodiments will in turncomprise a holographic material layer sandwiched by two substrates oralternatively a holographic material layer sandwiched by the core layerand a further substrate. The embodiments of FIGS. 28-31 may be used toprovide curved or planar displays.

In the embodiment of FIG. 31 the display comprises an upper glass layer260 a low index layer 261, a layer 262 a grating layer 263 divided intoan input grating 264 and a lossy output grating 265 which extracts lightuniformly along the waveguide to provide an expanding exit pupil and alower glass layer 266. The grating layer has an average refractive indexsubstantially identical to that of the core. In one embodiment the lowindex layer is an adhesive material of refractive index 1.315 in thevisible band. A typical ray path 1240-1242 is shown. In one embodimentthe upper and lower glass layers 260, 266 and the layer 262 each have arefractive index of approximately 1.5. An input ray is coupled into aTIR path inside the waveguide by the input grating and is eventuallycoupled out of the waveguide by the output grating. The structure asillustrated comprises a section of a windscreen. Typically, the overallthickness is around 4.4 mm. with the upper glass layer being equal inthickness to the waveguide stack comprising layers 261-266. A typicalray path is illustrated by 1270-1275. Noted the extraction of light bythe output grating only takes for downward propagating rays; the Braggcondition is not met by the upward propagating rays.

FIG. 32 illustrates an embodiment of the invention using the waveguideof FIGS. 28-31. The transparent display device 270, which may form partof a HUD or HMD, comprises a waveguide 271 into which are recordingthree gratings: an input grating 273 a fold or turning grating 274 andan output grating 275. The embodiments of FIGS. 28-31 represent a crosssection of the embodiment of FIG. 32. The waveguide also contains a thinbeamsplitter homogenizer layer for vertical and horizontalhomogenization. The input grating has high efficiency for maximum lightin-coupling efficiency. The output grating is a lossy grating forextracting light uniformly out of the waveguide along the beam path inthe waveguide. In contrast to the input and output gratings whichdeflect light in a plane orthogonal to the waveguide the fold gratingdeflects light substantially in the plane of the waveguide. Theformation of the image viewed from the eye box takes place in fourstages. In the first stage the input coupler couples image light 1280from an input image generator 272 into a vertical TIR path generallyindicated by 1281 within the waveguide. Typically, the image generatorwill further comprise an illumination source such as a laser or LED, amicrodisplay panel and a collimating lens. Advantageously, the inputcoupler grating uses a vertically rolled K-vector. The K-vector is avector aligned normal to the grating planes (or fringes) whichdetermines the optical efficiency for a given range of input anddiffracted angles. Rolling the K-vectors allows the angular bandwidth ofthe grating to be expanded without the need to increase the waveguidethickness. The input grating has a horizontal aperture width largeenough to mitigate horizontal illumination non-uniformity, commonlyreferred to as banding (resulting from the gaps that appear between theTIR beam paths after several bounces within the waveguide).Laser-illuminated waveguide displays are particularly susceptible tobanding. In the second stage the fold grating and homogenizingbeamsplitter together provide first axis of pupil expansion directinglight into a TIR path 1283 in the output grating. Note that a singlefold grating is used to support the field of view. In the third stagethe output grating, which is a lossy grating, provides uniform outputcoupling along the waveguide thereby providing the second axis of pupilexpansion. Finally, in the fourth stage collimated light generallyindicated by 1284-1287 is output towards the eye box 1288 from which acollimated image of the full field of view may be viewed.

In one embodiment the apparatus of FIG. 32 is embedded in a vehiclewindscreen. In such applications, the substrate indices used in thewaveguide stack should be close the index currently used in windscreenglass, that is, around 1.5. The inventors propose to use a modified PVBwith a slightly higher index than the PVB material currently used inwindshields (typically 1.48-1.50). A higher index is desirable to keepthe diffraction angle at an acceptably low level during holographicrecording.

In the above-described embodiments in which GRIN waveguides are combinedwith grating waveguides that are not embedded within a GRIN structuresthe embodiments of FIGS. 28-31 may be used to environmentally isolatethe grating waveguides.

The embodiments of FIGS. 28-31 may be applied to any of the waveguidedisplay devices disclosed in PCT Application No. GB2012/000677 entitledWEARABLE DATA DISPLAY, U.S. Pat. No. 8,233,204 entitled OPTICALDISPLAYS, U.S. patent application Ser. No. 13/317,468 entitled COMPACTEDGE ILLUMINATED EYEGLASS DISPLAY, U.S. patent application Ser. No.13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patentapplication Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAYand may benefit from the teachings provided therein.

Although the invention has been discussed in relation to a near eyedisplay it should be apparent from consideration of the drawings thatthe invention may also be used in other displays such as Head UpDisplays. The apparatus may also be used to provide an illuminationsystem. By reversing the light paths the apparatus may also be used inan image sensing system. A further application of the invention is in animage delivery system for providing a secondary image source for use inmicrolens array light field display. The invention may also be appliedto waveguide sensors such as eye trackers and fingerprint sensors.

A display according to the principles of the invention may include awaveguide despeckler based on principles disclosed in PCT ApplicationNo. PCT/GB2013/000500 entitled WAVEGUIDE FOR HOMOGENIZING ILLUMINIATION,and U.S. Pat. No. 8,224,133 entitled LASER ILLUMINATION DEVICE both ofwhich are incorporated herein by reference in their entireties.

It should be emphasized that the drawings are exemplary and that thedimensions have been exaggerated.

Any of the above-described embodiments may be implemented using plasticsubstrates using the materials and processes disclosed in PCTApplication No. PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. Advantageously,the SBGs are recorded in a reverse mode HPDLC material in which thediffracting state of SBG occurs when an electric field is applied acrossthe electrodes. An eye tracker based on any of the above-describedembodiments may be implemented using reverse mode materials andprocesses disclosed in the above PCT application.

The method of fabricating the SBG pixel elements and the ITO electrodesused in any of the above-described embodiments of the invention may bebased on the process disclosed in the PCT Application No. US2006/043938,entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A waveguide display, comprising: an input imagegenerator providing image light projected over a field of view; awaveguide having first and second external surfaces; and at least onegrating optically coupled to said waveguide for extracting light fromsaid waveguide towards a viewer of said display, wherein said waveguidehas a lateral refractive index variation between said external surfacesthat prevents rays propagating within said waveguide from opticallyinteracting with at least one of said external surfaces.
 2. Theapparatus of claim 1 wherein said waveguide contains a GRIN mediumwherein said grating is disposed in proximity to one of said externalsurfaces.
 3. The apparatus of claim wherein said waveguide contains aGRIN medium, wherein said grating is disposed within said GRIN medium.4. The apparatus of claim 1 wherein said waveguide contains a GRINmedium, wherein said grating is a surface relief structure formed on oneof said external surfaces.
 5. The apparatus of claim 1 wherein saidwaveguide comprises a first waveguide portion containing a GRIN mediumabutting a second waveguide portion operating in TIR and containing atleast one grating for extracting light from said second waveguideportion towards a viewer of said display.
 6. The apparatus of claim 1wherein said waveguide further comprises an input grating.
 7. Theapparatus of claim 1 immersed in air.
 8. The apparatus of claim 1providing one of a HUD, HMD or a light field display.
 9. The apparatusof claim 1 wherein said waveguide is curved.
 10. The apparatus of claim1 wherein said waveguide comprises at least one GRIN waveguide portionoptically coupled to at least one TIR waveguide portion, each said TIRwaveguide portion containing at least one grating.
 11. The apparatus ofclaim 1 wherein said waveguide comprises a stack of GRIN waveguidesoptically coupled to a stack of TIR waveguides, each said TIR waveguidecontaining at least one grating.
 12. The apparatus of claim 1 whereinsaid waveguide is immersed in a low refractive index external medium andcomprises a high refractive index core sandwiched by a low refractiveindex clad layer and at least one grating layer, wherein TIR takes placebetween the interface of said core layer and said grating layer and theinterface of said grating layer and said external medium.
 13. Theapparatus of claim 12 wherein said core and said grating layer havesubstantially same average refractive index.
 14. The apparatus of claim12 wherein said core has a refractive index greater than said gratinglayer average index.
 15. The apparatus of claim 12 further comprisinglow refractive index layers overlaying at least one of said gratinglayer and said low refractive index clad layer.
 16. The apparatus ofclaim 12 wherein said grating layer comprises a grating sandwiched bytransparent substrates, said grating layer and said substrates havingsimilar refractive indices.
 17. The apparatus of claim 1 wherein saidgrating layer comprises an input grating and an extraction grating. 18.The apparatus of claim 1 further comprising a beamsplitter layer. 19.The apparatus of claim 1 wherein said image light is collimated.
 20. Theapparatus of claim 1 wherein said grating is one of a Bragg Grating, asurface relief grating or a switchable Bragg grating recorded in a HPDLCmaterial, a uniform modulation HPDLC material or a reverse mode HPDLCmaterial.